The mouse atonal homolog 1 (Math1) encodes a basic helix-loop-helix transcription factor that is expressed in the progenitors of the inner ear hair cells, cerebellar granule cells, spinal cord dorsal interneurons, several brain stem nuclei, merkel cells, and the secretory cells of the intestine. Of note also is that the expression of Math1 and HATH1 is upregulated in medulloblastomas from mouse models and human samples, respectively. Gene targeting has demonstrated that Math1 is necessary for the genesis of many of these cells. However, some of these cell populations differentiate postnatally, and because Math1 null mice die at birth from an inability to breathe, the fate of these cells and their dependence on Math1 function have not been fully analyzed. To study the fate of all Math1-expressing cells, we have targeted an inducible Cre recombinase to the Math1 genomic locus. When these mice are crossed to a reporter strain, this Cre/loxP system will permanently label Math1-expressing cells such that their entire downstream lineage can be traced into adulthood. We will also use an existing conditional allele to delete Math1 in the brainstem and cerebellum at specific embryonic and postnatal time points in order to identify the brainstem nucleus required for the respiratory defect in the Math1 nulls and to study the role of Math1 in cerebellar granule cell proliferation and differentiation. The combination of these fate-mapping and conditional knock-out studies should provide basic knowledge about the development of Math1-expressing cell types, the initiation of breathing, and the connection between neuronal progenitor proliferation and differentiation. This information may provide insight into hindbrain diseases such as Medulloblastomas, and perinatal respiratory dysfunction such as Sudden Infant Death Syndrome. [unreadable] [unreadable]